Experimental

Question 1

What is the concentration of ion in the trap and how does it compare to solutions in other XAS experiments?

Answer 1

• Constant target density in the trap is ensured by continuously filling the trap up to the space charge limit.
• cluster ion density in the trap is ≈ 5 × 10⁸ cm⁻³
• Working concentration ≈ 8.3 × 10⁻¹⁶ mol/L
• Ronny in liquid jet > 100 mmol/L
• length of the trap = 200 mm, beam profile = 0.6 × 0.7 mm
• number of molecules interacting with beam ≈ 1.3 × 10⁸ molec.
• at LiXEdrom: flat jet ≈ 1-4 µm thick, beam profile 100 × 25−150 µm → ≈ 4.7 × 10¹² molec.

Question 2

What is the top-up mode in an operating synchrotron and how does Bessy II operate?

Answer 2

• Top-up is a mode of operation that aims to maintain a steady current in the storage ring by periodically injecting small amounts of current.
• Bessy II currently usually operates in a standard fill pattern with multi bunch hybrid.
• Usual multibunch filling: current ≤ 300 mA; multi-bunch-train = 0.9 mA; bunch length (FWHM) = 45 ps. Further modes in image.

Question 3

How the X-ray absorption affects the concentration of precursor and product ions in the trap?

Answer 3

• when absorption occurs, precursor ions density is reduced to n ≈ 5 × 10⁷ cm⁻³ (q reduces to ≈ 0.05 while product q-parameter becomes ≈ 0.5)
• Typical cross sections of resonant excitation at the L edge of 3d transition metals of 10 Mbarn per atom; 10¹² photons s⁻¹ yield approximately 1 × 10⁵ product ions per second
• Product ions created in the ion trap can also absorb x-ray photons → contributes about 0.1% to the x-ray absorption spectrum (neglectable)

q describes the trapping efficiency; q = 0.5 for good trapping conditions

Question 4

How is it assured that only cationic species enter the trap?

Answer 4

• Iris aperture: in general. neutral species will exit the SRC in the direction normal to the iris wall (flow dynamics), which has an inclination. In this step, the neutral species will be guided to the upper part of the wall around the hexapole, as the latter is optimized for cations.
• Optics: voltages are optimized for cations, getting rid of most anions
• Deflector: the 90° deflector prior to the ion trap is optimized for cations, guaranteeing only those enter the trap

Question 5

What does the measured signal consist of?

Answer 5

• We measure voltage pulses of the ion bunches being extracted from the trap → TOF-MS
• This signal is integrated across the energy range → XAS spectrum
• The spectrum is normalized by the photonflux, which is obtained considering the quantum efficiency of the photodiode → that is because not all photons are converted to current by the diode → convert photocurrent to photonflux → quantum efficiency QE = Ne/Nν

Question 6

If you could build a new machine or improve yours to complement your investigations, what modifications would you make?

Answer 6

• implement tandem MS with reaction cell in between, to investigate selected products of reactions of interest, such as hydrogen atom abstract from methane
• install laser vaporization source to compare if there is a difference in the produced isomers

Question 7

Why a quadrupole ion trap, and not a higher order, e.g. hexapole or octopole?

Answer 7

• V_eff in the quadrupole ion trap is proportional to r², in the haxapole ~r⁴ and octopole ~r⁶. This means that the potential becomes flatter in the center leading to less particle density as the order of the poles increases, as there is little electric field pushing the particles to the center.
• To assure the beam is interacting with the most particles as possible, ideally the pearticles are concentrated in the center
• According to space charge limit, a radial charge distribution follows ρ∝r^k-4 (k=number of poles), and only the quadrupole ion trap has a non-zero charge as r→0

Question 8

What is the temperature of the ions in the trap and what is the equivalent energy?

Answer 8

E = m c² = k T
T_trap ⁓ 10 K
k = 1.380649×10 ⁻²³ J/K = 8.6173303×10^-5 eV/K

E_trap ⁓ 8.62×10^-4 eV = 0.0832 kJ/mol

Question 9

Why do you measure ion yield mode and not fluorescence or absorption directly (transmission mode)?

Answer 9

• For X-ray absorption experiments of diluted samples, e.g., gas-phase samples, the measurements in transmission mode turn out as very challenging due to a low absorption cross section.
  Beer-Lambert law: I = I₀ e ^-μl
  where the absorption coefficient μ is proportional to the absorption cross-section σ, which is proportional to the concentration of the sample. Since the concentration is very low, it’s hard to obtain μ.
• The absorption could be measured indirectly through secondary processes that follow the absorption: fluorescence or ion yield spectroscopy.
• The fluorescence probability is given by the dipole matrix element 〈c|r|v〉 ∝ E³ ≈ Z⁶ (|c〉 core state, |v〉 valence state)
• the probability of the Auger-Meitner decay (non-radiative) depend on the wave-function overlap
• For a given pair of states the overlap does not vary strongly with Z. Hence, which process is dominant strongly depends on the atomic number Z and the states involved → AM = 1-F
• At the energy regimes that we work, the Auger-Meitner yield is always higher than fluorescence yield

A particle in a core excited state can relax into energetically lower lying states by transferring an electron to the empty core state. The energy can either be emitted by a fluorescence photon or transferred to an other electron if the energy gained is sufficient for a transfer of the electron into a continuum state For particles consisting of more than one atom the charge present after multiple Auger decays can lead to fragmentation due to Coulomb repulsion of charges on the particles atoms.

Question 10

How do you overcome the challenge of measure gas-phase samples?

Answer 10

For X-ray absorption of gas-phase samples, naturally very challenging due to a low absorption cross section. In our experimental setup, the particle density is even attenuated by the prior size selection. To overcome this challenge, a high X-ray photon flux, such as provided by synchrotron radiation facilities (10¹² photons per second), can be used and the particle density can be improved by accumulation within the interaction zone with the X-rays beam, to the charge density limit (5×10⁸ cm^-3 ).

The maximum charge density ρ in a linear quadrupole trap is given by ρ = (ε₀ V₀ q)/r₀²

Question 11

Briefly explain the principles of a time-of-flight mass spectrometer.

Answer 11

• Ions are accelerated by an electric field, giving them the same kinetic energy (mv²/2)
• The ions travel through a flight tube of a fixed length.
• Lighter ions travel faster and reach the detector earlier, while heavier ions travel slower and arrive later
• The time of flight (t) is related to the mass (m) of the ion by the equation:
  m = At²
  where A is a constant that depends on the instrument’s parameters.
• By measuring the flight time accurately, the mass of the ion can be determined with high precision

Question 12

What are the typical parameters you had for the clusters production at the magnetron sputter source.

Answer 12

The discharge power used to obtain the [MOn]+ species was typically about 4 W. The reactive and buffer gas composition was adjusted by mass flow controllers with the optimized values of 10 sccm of argon, 20 sccm of helium buffer gas and 100 sccm of the 1% oxygen/helium mixture, at a pressure of typically 0.3–0.9 mbar in the aggregation pipe.

Question 13

Briefly explain how the ion trap works.

Answer 13

As the particles enter the trap, the potential at the aperture is kept in such an amplitude low enough to allow the particles to enter, but still high enough to promote the trapping. The static potential inside the ion trap is a superposition of a static potential of the four quadrupole electrodes and the triangle-shaped potential created by the side electrodes placed between the electrodes. The triangle creates an effective linear potential, which pushes the particles towards the exit aperture and the ion bunches can be extracted by lowering the potential of the exit aperture.
Helium is introduced in the ion trap as a buffer gas and is responsible for slowing down the charged particles via collisions, improving the trapping of the ions. Helium gas flow into the chamber is adjusted to keep the pressure in the chamber’s wall in the order of 7 × 10^-7 mbar. The helium buffer gas also thermalizes the particles and rf heating has negligible influence on the particles’ temperature.

Question 14

Briefly explain how a quadrupole mass filter works

Answer 14

• The four rods are arranged in a square configuration, with opposite pairs connected electrically
• A combination of radio frequency (RF) and direct current (DC) voltages is applied to the rod pairs, creating an oscillating electric field
• Ions travel between the rods, and their trajectories are affected by the electric field
• For specific RF and DC voltage combinations, only ions with a particular m/z ratio will have stable trajectories and pass through the filter
• Ions with unstable trajectories will collide with the rods and be filtered out
• By varying the RF and DC voltages while maintaining a constant ratio between them, the quadrupole can selectively allow ions of different m/z ratios to reach the detector
• The x-direction acts as a high-pass mass filter, allowing only higher masses to pass, while the y-direction acts as a low-pass filter, allowing only lower masses to pass
• The combination of these two directions creates a bandpass filter capable of resolving individual atomic masses